Twenty-five years ago, the Clean Air Act turned down the exhaust fumes coming from south of the border.

The sweeping amendments passed in 1990 to the American legislation sought to address acid rain, ozone depletion and toxic air emissions resulting from industry, primarily those burning fossil fuels.

And it was largely successful; acid rain-causing emissions have been reduced by upwards of 85 per cent in the United States, Eddie Halfyard, a research scientist at the University of Windsor, said Wednesday.

Yet high levels of acidity continue to be one of the main factors keeping salmon out of all but a handful of the 65 rivers on this province’s Eastern Shore in which the fish used to spawn.So what gives?

“When all that acid rain fell really heavy through the ’80s, it stripped the soil of compounds like calcium and magnesium that would buffer its effects,” said Halfyard, who lives in Beaver Bank and volunteers with an effort to compensate for the damage done by acid rain to the West River, near Sheet Harbour.

The Nova Scotia Salmon Association and the Eastern Shore Wildlife Association operate the only lime doser in North America. As one might lime soil to adjust the pH level, the groups have been liming the West River since 2005.

They’ve had success.

Before the project began, only about 3,000 juvenile salmon were being produced annually. That’s up to about 11,000.

With high at-sea mortality rates, the causes of which still aren’t fully understood, all those smolts manage to keep salmon runs going in the river but they are still below historic levels.

The equipment is also expensive, running about $100,000 per unit.

A few decades of heavy acid rain was able to do so much damage to the Eastern Shore because the area only developed a thin layer of soil since the glaciers retreated about 10,000 years ago. The rock below that soil is primarily granite, which is hard and only very slowly ground into dirt by the slow work of plants and erosion.

Meanwhile, rivers running into the Northumberland Strait and Gulf of St. Lawrence have recovered quicker because they are surrounded by deeper soils that have retained their ability to buffer the acid rain that still falls.

Halfyard said research predicts that if all acid rain stopped, it would take 70 to 100 years for erosion to free up enough calcium on the Eastern Shore to buffer against it.

“It is a bit of a bleak outlook.”

But there’s another experiment going on in Nova Scotia to keep rivers healthy until nature can heal the damage.

A group led by Dalhousie University’s Shannon Sterling has been experimenting with spreading soil pH modifiers onto the upland from rivers instead of directly into the water itself.

“With watershed liming, if you do it right — and we’re all trying to figure out what right is — you can have a long-term impact for, say, 25 or more years,” said Lewis Hinks, Nova Scotia director for the Atlantic Salmon Federation.

It’s about buying time — albeit a lot of time — until the damage done in the decades before the Clean Air Act can be reversed by nature.

And, adds Charles Clattenburg, it’s not just about salmon.

“What we’re finding is if you can keep the rivers healthy for salmon, you can keep them healthy for all species,” said the president of the Eastern Shore Wildlife Association.

“So far locally, we do have reports of anglers having more success with other species like trout and sea trout.”

The association’s volunteers installed the West River’s first counting fence for mature salmon this summer. Ultimately, it should tell them how successful their efforts have been.